chuck question

[Ray C]

I'm going to toss something out here that may sound unusual. I'm also embedding some tips along the way.

1) TIP: First, rig-up a balancing mechanism (I can provide demonstrations) and find the heavy side of both the backplate and chuck. For the chuck, mount a bar in the jaws and set it on bearings. When you re-assemble things, put the heavy sides opposite of eachother and the "average" balance will be closer to neutral.

2) I don't put shoulders on my 3 jaw chuck backplates that are designed to retain the chuck. In other words, I push the diameter of the shoulder back about 1/16" so it is not a tight fit with the inside collar of the chuck. I make sure the backplate is flat (with a face cut) on the shoulder area. I take the jaws out of the chuck and I surface grind the back of the chuck so it's flat. Finally, lightly attach the chuck with the bolts to the backplate and lathe then, mount a proof rod in the jaws and tap the chuck body until it's centered using a DI about 1" from the jaws. Now tighten the bolts. -Done. With the bolts properly tightened to roughly 20ftlbs torque, that chuck will not move and if it does, it's doing you a favor by giving way before your bearings and spindle take a pounding.

3) With a proof rod in the jaws, move the DI out about 4" from the jaws. If you're seeing a large difference (greater than 0.004"), then the jaws need to be tested and possibly adjusted.

4) Once a chuck is mounted and aligned nicely, it's always a pleasure to work with a balanced chuck. I always take the time to do it. Read-up on the acceptable procedures before you try lest you make a doorstop out of your chuck.



Ray

 

[Bill C.]

I'm going to toss something out here that may sound unusual. I'm also embedding some tips along the way.

1) TIP: First, rig-up a balancing mechanism (I can provide demonstrations) and find the heavy side of both the backplate and chuck. For the chuck, mount a bar in the jaws and set it on bearings. When you re-assemble things, put the heavy sides opposite of eachother and the "average" balance will be closer to neutral.

2) I don't put shoulders on my 3 jaw chuck backplates that are designed to retain the chuck. In other words, I push the diameter of the shoulder back about 1/16" so it is not a tight fit with the inside collar of the chuck. I make sure the backplate is flat (with a face cut) on the shoulder area. I take the jaws out of the chuck and I surface grind the back of the chuck so it's flat. Finally, lightly attach the chuck with the bolts to the backplate and lathe then, mount a proof rod in the jaws and tap the chuck body until it's centered using a DI about 1" from the jaws. Now tighten the bolts. -Done. With the bolts properly tightened to roughly 20ftlbs torque, that chuck will not move and if it does, it's doing you a favor by giving way before your bearings and spindle take a pounding.

3) With a proof rod in the jaws, move the DI out about 4" from the jaws. If you're seeing a large difference (greater than 0.004"), then the jaws need to be tested and possibly adjusted.

4) Once a chuck is mounted and aligned nicely, it's always a pleasure to work with a balanced chuck. I always take the time to do it. Read-up on the acceptable procedures before you try lest you make a doorstop out of your chuck.



Ray

THANKS everyone for all your advise. I never mounted a chuck to a backplate but I knew that chuck needed help. Makes me appreciate all those who have mounted or repaired the chucks I have used in the past. Bill

 

[aforsman]

Thanks to all for the suggestions. I kind of like Ray's idea of just getting the chuck centered on the backplate and tightening the bolts enough to keep it there for no other reason than it's the easiest one to implement ). It also seems intuitive to me that the chuck should seat on the outer ring as Sharon mentioned, otherwise the bolts would tend to pull and warp the assembly when tightened. I don't have a surface grinder, although a local machine shop could probably do it for a few bucks. I have a mini-mill and could take some light surface cuts followed by some lapping on a granite surface plate (which I have), but I'm sure that wouldn't be nearly as accurate. I think I will proceed in stages - first by bringing the inner face down enough to be sure the chuck seats on the outer face and cutting the shoulder back a few thousandths to give some wiggle room. Then I'll follow Ray's centering procedure and see where it puts me. If it's good enough, I'm done. If not, I'll look at getting it ground.

The only thing I don't really get is the balancing. Unless the chuck and/or faceplate are really out of balance and you're running at very high speeds I wouldn't think it would even be noticeable. Can you elaborate a little more on what the balancing does from a practical standpoint?

Allen

 

[awander]

The chuck needs to "seat" in a couple of different ways.

The back of the chuck itself should seat on the outer "ring" of the backplate, as it is moved in the negative Z direction(towards the headstock).

The inner, raised center portion of the backplate(some call this the spigot) should NOT contact the back of the recess in the chuck.

The outside diameter of the raised portion of the backplate should seat tightly inside the diameter of the recess in the back of the chuck(this is what centers the chuck on the X axis.


...and you do know the proper designations for the axes, right? X is the movement of the cross-slide, positive towards you, and negative away from you(until the tool tip is at center of rotation, or X=0). Z is the carriage movement, positive as you move towards the tailstock.

 

[Ray C]

First I just want to mention there are different techniques and preferences so, by no means am I suggestion the method I mentioned is the best or only way. -Many valid ways of skinning the cat and the important thing is that you end up with a good chuck at the end of the day.

Balancing. If you run your machine at all speeds (with the jaws closed) and if you neither see or feel any vibration. No action is necessary. An unbalanced chuck vibrates and it shows-up in the work piece quite visibly...

If you're so inclined, have a look here as there are some sections that discuss chuck setup and balance.

http://www.hobby-machinist.com/show...surement-and-Setup?highlight=D1-4+Measurement


Ray

Thanks to all for the suggestions. I kind of like Ray's idea of just getting the chuck centered on the backplate and tightening the bolts enough to keep it there for no other reason than it's the easiest one to implement ). It also seems intuitive to me that the chuck should seat on the outer ring as Sharon mentioned, otherwise the bolts would tend to pull and warp the assembly when tightened. I don't have a surface grinder, although a local machine shop could probably do it for a few bucks. I have a mini-mill and could take some light surface cuts followed by some lapping on a granite surface plate (which I have), but I'm sure that wouldn't be nearly as accurate. I think I will proceed in stages - first by bringing the inner face down enough to be sure the chuck seats on the outer face and cutting the shoulder back a few thousandths to give some wiggle room. Then I'll follow Ray's centering procedure and see where it puts me. If it's good enough, I'm done. If not, I'll look at getting it ground.

The only thing I don't really get is the balancing. Unless the chuck and/or faceplate are really out of balance and you're running at very high speeds I wouldn't think it would even be noticeable. Can you elaborate a little more on what the balancing does from a practical standpoint?

Allen

- - - Updated - - -

First I just want to mention there are different techniques and preferences so, by no means am I suggestion the method I mentioned is the best or only way. -Many valid ways of skinning the cat and the important thing is that you end up with a good chuck at the end of the day.

Balancing. If you run your machine at all speeds (with the jaws closed) and if you neither see or feel any vibration. No action is necessary. An unbalanced chuck vibrates and it shows-up in the work piece quite visibly... A good way to gauge this is to run your machine w/the chuck removed at all speeds then, compare how things feel doing the same thing with the chuck installed.

-BE SAFE: Chuck jaws closed. Don't run in reverse at high speed. Make sure the chuck is securely bolted to the plate and screwed to the spindle.


If you're so inclined, have a look here as there are some sections that discuss chuck setup and balance.

http://www.hobby-machinist.com/show...surement-and-Setup?highlight=D1-4+Measurement

Ray

 

[aforsman]

Done!!! As per my plan, I cut the runout of the outer face and turned the shoulder in about .010". I also cut the inner face down about .020" to give clearance in the center. With just a little bit of bumping, I was able to get it to run within 1-1/2 thou an inch from the chuck. I tried it a foot or so out and there was much more runout, but I don't really trust that the piece of stock I'm using is that true. I'll play with it a little more tomorrow, but as of this evening I'm quite happy ). I plan on reading the thread on balancing in case I find the need to do it. Thanks again to all.

Allen

 

[Ray C]

Good, I'm glad it worked.

Before you claim victory, remove the jaws and place a DI against the face of the chuck. Make sure you don't have wobble. Unless someone butchered-up the chuck, it will probably be fine.

I'm going to assume the Allen cap screws are 5/16 or 3/8" bolts. Clean and lightly oil the bolts and internal threads then, if you have a torque wrench, tighten 5/16 to 19ftlb and 3/8 to 30ftlb. Call it a day. Under normal use, the chuck will not budge one bit. If you ever accidentally bang the chuck, with any luck, the chuck will be allowed to shift due to the shoulder diameter clearance and that just might save some stress on the bearings, spindle etc... BTW: I used to align 3 jaw chucks by getting the shoulder diameter to fit perfectly but one day, I had a self revelation, tried it this way and it worked. I have three, 6" 3 jaw chucks and they are fine. BTW, the procedure is different for 4 jaw. With 4 jaw, I still don't fit tightly to the shoulder but, I center the body by reading with a DI on the outside of the body itself.

I would not bother to check runout much further than 4" from the chuck. If you read a considerable amount at that distance, it means the jaws may need to be squared-up. Let us know how that goes when you get a more trustworthy proof rod.

Ray

Done!!! As per my plan, I cut the runout of the outer face and turned the shoulder in about .010". I also cut the inner face down about .020" to give clearance in the center. With just a little bit of bumping, I was able to get it to run within 1-1/2 thou an inch from the chuck. I tried it a foot or so out and there was much more runout, but I don't really trust that the piece of stock I'm using is that true. I'll play with it a little more tomorrow, but as of this evening I'm quite happy ). I plan on reading the thread on balancing in case I find the need to do it. Thanks again to all.

Allen

 

[Ray C]

... Just got an offline inquiry about the chuck possibly shifting w/o a tight shoulder fit...

A 5/16" bolt torqued to 19ft/lb has about 3000lbs of clamping force. There are three bolts in most 6" chucks therefore, there's 9000lbs of total clamping force between the contact areas. The area of a circle is Pi x R[SUP]2[/SUP]. If you assume the shoulder contact area is 3/4" wide (sounds about right) and calculate the area of contact, with a 6" chuck, you get Pi x 9 - Pi x 7 = 6.28 sq inches. Thus, the force between the contact area is 9000/6.28 = 1432 PSI. If you've got 3/8" bolts the number is way more... Should be just peachy. If you put more than 9000 pounds of lateral pressure on your chuck, you got other problems to deal with... -like replacing your compound or cross-slide.


Ray

 

[awander]

... Just got an offline inquiry about the chuck possibly shifting w/o a tight shoulder fit...

A 5/16" bolt torqued to 19ft/lb has about 3000lbs of clamping force. There are three bolts in most 6" chucks therefore, there's 9000lbs of total clamping force between the contact areas. The area of a circle is Pi x R[SUP]2[/SUP]. If you assume the shoulder contact area is 3/4" wide (sounds about right) and calculate the area of contact, with a 6" chuck, you get Pi x 9 - Pi x 7 = 6.28 sq inches. Thus, the force between the contact area is 9000/6.28 = 1432 PSI. If you've got 3/8" bolts the number is way more... Should be just peachy. If you put more than 9000 pounds of lateral pressure on your chuck, you got other problems to deal with... -like replacing your compound or cross-slide.


Ray

I don't think it is quite that simple, Ray.

Just because there is 9000 lbs of force holding the chuck on, doesn't mean it would take 9000 lbs of lateral force to move the chuck. It all depends on the friction between the chuck and the backplate.

Think of a 9000 lb weight sitting on asphalt. There is 9000 lbs holding it in place, and it will be very hard for someone to push/move it sideways. But put it on ice, and you could likely push it and move it pretty easily.


Also, being picayune, I just have to point out that your post above contains an error in terminology, when you wrote, "the force between the contact area is 9000/6.28 = 1432 PSI". Force is not measured in PSI, PSI is a measurement of pressure. I know that is what you meant, but I feel the need to correct it for others who may not know.

 

[Ray C]

It's even more complicated than that. The phenomenon you mentioned is called static and and dynamic friction but that only applies to unconstrained motion. In this case, the bolts are constraining the chuck so, it would take a minimum of 9000lbs (to begin overcoming the static friction and then overcome the bolt tension) to move the chuck. My point was to outline the general principals involved and get folks to realize that you'd need to put the weight of two automobiles on the chuck before anything could possibly happen. I also made a light attempt to show folks how to make simple clamping force calculations. I should have mentioned that the values are taken from a standard chart.

It's pretty common when describing a problem to use the word "force" as an improper noun or as an adverb. The units of the final value take precedent and clear-up any confusion. Many folks say things like "The force of the crash caused damage to the vehicles". -It makes perfect sense but it's wrong. Collision encounters always generate impulses -not forces. -But we still understand what is meant.

Ray

I don't think it is quite that simple, Ray.

Just because there is 9000 lbs of force holding the chuck on, doesn't mean it would take 9000 lbs of lateral force to move the chuck. It all depends on the friction between the chuck and the backplate.

Think of a 9000 lb weight sitting on asphalt. There is 9000 lbs holding it in place, and it will be very hard for someone to push/move it sideways. But put it on ice, and you could likely push it and move it pretty easily.


Also, being picayune, I just have to point out that your post above contains an error in terminology, when you wrote, "the force between the contact area is 9000/6.28 = 1432 PSI". Force is not measured in PSI, PSI is a measurement of pressure. I know that is what you meant, but I feel the need to correct it for others who may not know.